mips: export pci_iounmap()

I added this function in an earlier patch, but the missing export caused
a build failure

Reported-by: Nathan Chancellor <[e-mail]>
Signed-off-by: Arnd Bergmann <[e-mail]>

arch/mips/lib/iomap-pci.c

···
 	if (addr < base || addr > (base + resource_size(ctrl->io_resource)))
 		iounmap(addr);
 }
+EXPORT_SYMBOL(pci_iounmap);
 
 #endif /* CONFIG_PCI_DRIVERS_LEGACY */